Little Rock, Arkansas (CNN) -- Arkansas Sen. Blanche Lincoln won the Democratic primary Tuesday, beating back a challenge from Lt. Gov. Bill Halter in a high-profile contest that exposed a rift between the liberal and centrist wings of the Democratic Party.

Lincoln was targeted by national unions and liberal activists, who accused the Arkansas Democrat of turning her back on them as well as failing to support President Obama's policy goals.

The Lincoln critics found their candidate in Halter and poured millions of dollars into Arkansas. Halter officially announced his candidacy in March making the primary a sprint, not a marathon.

With the help of these outside activists, the lieutenant governor forced Lincoln into a June runoff election after she failed to receive more than 50 percent of the vote in May's primary.

Heading into Tuesday, some analysts were predicting that Lincoln would become the third incumbent senator to fall this year.

But over the past month, Lincoln hit the campaign trail in a very aggressive way, staring down the union and liberal opposition and proclaiming at each turn that she was an independent voice for the state.

"The vote of this senator is not for sale and neither is the vote of the people of Arkansas," Lincoln said Tuesday night as she claimed victory.

In November, Lincoln will face Rep. John Boozman who won the Republican primary last month.

Lincoln ran a classic inside-outside primary campaign as she touted her seniority on Capitol Hill as chair of the Agriculture, Nutrition & Forestry Committee, while declaring she was not beholden to Washington's special interests. Lincoln has served two terms in the Senate and two terms in the House.

She also called in former President Bill Clinton to help make the closing case to Democratic voters that she deserved a shot at a third term. Clinton, a former Arkansas governor, campaigned for Lincoln and she aired a TV commercial emphasizing his endorsement.

In a fundraising email sent shortly after Lincoln was declared the winner, the Democratic senator wrote that Clinton had called to congratulate her.

"President Clinton called me tonight when it was clear we were going to win and said, 'Blanche, you're the new Comeback Kid!'" Lincoln wrote.

Even though they failed to defeat Lincoln, two major unions stood by their decisions to challenge her and suggested that this race served as a warning shot to other Democrats that organized labor will not blindly support candidates.

"While tonight was not our preferred outcome, it was still a tremendous victory for working families," AFL-CIO President Richard Trumka said in a statement. "Taking a two-term incumbent in deep-red Arkansas into a runoff, and coming within few thousand votes ... is a virtually unprecedented achievement. If working families were able to accomplish this in Arkansas, imagine what they can achieve in other states."

"It is also now abundantly clear to all politicians that if they want to get the support of working families on Election Day, they are going to need to fight for their issues every day," he added.

Service Employees International Union President Mary Kay Henry expressed a similar sentiment in a separate statement.

"The tens of thousands of volunteers and activists have made their point loud and clear: If you stop fighting for working families, working families will stop fighting for you," she said.